fix: improve extension runtime safety, HTTP response URL, SongLink parsing, and recommended service for extensions

- Add 'url' field (final URL after redirects) to all extension HTTP responses and fix fetch polyfill to return final URL instead of original request URL
- Fix RunWithTimeout race condition: increase force-timeout from 1s to 60s to prevent concurrent VM access crashes, add nil guards
- Use lockReadyVM() for thread-safe VM access in GetPlaylistWithExtensionJSON and InvokeAction
- Handle mixed JSON types (string, null, array) in SongLink resolve API SongUrls field
- Fix recommended download service not showing for extension-based searches in download picker

lib/screens/home_tab.dart

@@ -757,7 +757,8 @@ class _HomeTabState extends ConsumerState<HomeTab>
           trackName: track.name,
           artistName: track.artistName,
           coverUrl: track.coverUrl,
-          recommendedService: trackState.searchSource,
+          recommendedService:
+              trackState.searchExtensionId ?? trackState.searchSource,
           onSelect: (quality, service) {
             ref
                 .read(downloadQueueProvider.notifier)